Television and like system



March 26, 1940 RWMLLANS HAL 2,194,514

TELEVISION AND LIKE SYSTEM Filed April 12, 1934 2 Sheets-Sheet 1 BLACK,8

Mom/m 16R 2; 4 BLHCK ,6 9 7 6 I REI/YSERTING AMPL we 1 OSCILLRTOR l4ELEc'r/P/cnL Z ZERO INVENTORS 36 as I B 53 3 PETER WILLIAM WILLANSQ 3WILLIAM SPENCER PERCIVAL, 1; ERIC LAWRENCE CASLING WHITE,& y T CHARLESPERCY OSBORNE, DE- A BY CEASED, I I v HILDA E. F. OSBORNE ADMINISTRATRIX,6. 6'. v

March 26, 1940. P. w. WILLANS r AL 2,194,514

TELEVISION AND LIKE SYSTEM Filed April 12, 1954 2 Sheets-Sheet 2AUXILIARY AMPLIFIER DEMODULATOR Mm MoouLA-roR\ MDDULl-ITDR M c AUXILIARY2 \Ag ARRIER Oscuumnz I stun-ran 4 SAw T0o'rH b2 GENERATOR 66 '11 NFL!PIER I INVENTORS 5 P ER WILLIAM WILLANS,

WILLIAM SPENCER PERCIVAL, ERIC LAWRENCE CASLING WH|TE,&.

- CHARLES PERCY OSBORNE, DE-

L Fig CEASED, BY

HILDA E. F. OSBORNE,ADMINISTRATRIX BY ATTORNEY Patented Mar. 26, 1940UNITED STATES TELEVISION AND LIKE SYSTEM Peter William Willans,

Hampstead, William Spencer Percival, Hanwell, Eric Lawrence CaslingWhite, Hillingdon,

and Charles Percy sborne, deceased, late of Hillingdon, England,

- by Hilda E. F. Osborne, don, England, assignors administratrix,Hillingto .Electric & Musical Industries Limited, Hayes, Middlesex,England, a company of Great Britain Application April 12, 1934, SerialNo. 720,206 In Great Britain April 13, 1933 15 Claims.

The present invention relates to television, picture transmitting andthe like systems of the kind in which signals representative of thebrightness of elemental areas of an object are transmitted together withsynchronising signals along a single channel which is usuallyconstituted by a radio frequency carrier wave. The synchronising signalsare often in the form of pulses, one pulse, known as a line impulse,being transmitted in the interval between the scanning of each strip ofthe object and one pulse, known as a frame impulse being transmitted inthe interval between two successive scannings of the object. The lineimpulses are usually made of different form from the frame impulses sothat the two sets of impulses can be separated readily at the receiver.

At the receiver the impulses may be used to control the frequency of ascanning device. In the case of a receiver embodying a cathode ray tube,each set of impulses is used to control the frequency of a generator ofoscillations of sawtooth wave form which serve in known manner todeflect the ray across the screen so that the ray scans the screen inclose parallel strips.

Where the composite signals transmitted, comprising picture signals andsynchronising impulses, are generated by means which do not producecomponents having frequencies down to 80 and including effectively zerofrequency or where the signal is passed through any device,'such as acondenser or a transformer, which suppresses the D. C. and low frequencycomponents of the signal, the composite signal, when plotted againsttime, will be seen to consist of fluctuations above and below a zeroline so situated that the average area enclosed by the fluctuationsabove the zero line is equal to the average area enclosed below theline. This zero line may be referred to as the electrical zero line and,even if the D. C. and low frequency signal components are transmitted,when the composite signals are received on re-- ceivers of known type,the electrical zero line will be the one about which the fluctuations ineffect 45 take place. This electrical zero'does' not represent any fixedvalue of picture brightness and if the average brightness of the objectchanges the brightness represented by the electrical zero also changes.A serious difficulty arises on account 50 of the fact that the amplitudeof the synchronising pulses with respect to the electrical zero linealso changes. Assuming that the synchronising pulses are in such a senseas to correspond to an increase in blackness they are usually arrangedto extend from a level corresponding to black in the opposite sense tothe picture signals, that is, they extend in what may be called ablacker than black direction. In this case the effect of an increase inaverage picture brightness is to increase the amplitude of thesynchronising 5 pulses with respect to the electrical zero and theeffect of a decrease in average brightness is to decrease the amplitudeof the pulses.

In order that accurate synchronising may be obtained, the synchronisingsignals must be separated from the picture signals by some suitabledevice such as a suitably biased rectifier. The bias of this rectifier,if no D. C. component is present in the signals, must be fixed relativeto the electrical zero line so that the rectifier will passsynchronising signals and not picture signals. Because the amplitude ofthe synchronising signals relative to the electrical zero line variesand also because the amplitude of the picture signals corresponding tothe blackest parts of the picture at any moment also varies from time totime, it is necessary to provide large synchronising signals which willunder all circumstances project sufficiently from the electrical zeroline to allow some at least of their amplitude to be reliably separatedwithout either the separated synchronising pulses being unduly smallduring conditions of minimum picture brilliancy, or there beinganychance of picture signals being passed to the synchronising channelduring conditions of maximum picture brilliancy. Thus the synchronisingimpulses transmitted must be given a relatively large amplitude, forexample two or more times the maximum picture minimum ampli- The use ofsynchronising signals of relatively large amplitude is of courseuneconomical in that a relatively large fraction of the total availablechange of carrier amplitude has to be used for the synchronisingsignals.

It is an object of the present invention to provide a television or thelike system in which the ratio of maximum be increased.

the separating means at the receiver, are such that, with respect tosome point in the receiver, the amplitude of the synchronising signalsfed to the separating means is substantially unaffected by changes inthe average amplitude of the picture signals. In other words it isarranged that the composite signal fed to the separating means containsthe appropriate components of low frequency down to effectively zerofrequency which will for convenience be referred to as the D. C.component. Thus any particular picture signal voltage applied to the.separating means with reference to some fixed point represents anabsolute value of picture brightness.

The present invention further provides a television or the like systemcomprising a transmitter adapted to transmit, by modulated carrier,composite signals comprising picture signals interspersed withsynchronising signals and a receiver for the signals, having means forseparating the synchronising signals from the picture signals, wherein,in the composite signals used to modulate the carrier at the transmitterthe ratio of the amplitude of the synchronising signals to that of themaximum picture signal is less than 1-5 and wherein, at the receiver,the circuits between the aerial or equivalent input terminals and theseparating means are such that the synchronising signals fed to theseparating means are of substantially constant amplitude with respect tosome point in the receiver.

The modulation at the transmitter is preferably effected in such a waythat the peaks of the synchronising pulses are represented bysubstantially zero carrier amplitude. One advantage of this is that thechance of producing false synchronising signals, in the form of pulses,by an interfering signal is reduced because if an interfering signal'ofsuitable wave form to produce a false pulse has an amplitude greaterthan a certain value it fails to reduce the carrier amplitudesubstantially to zero (as is required for the production of asynchronising pulse) and, instead, reverses the phase of the carrieroscillations. If the interfering signal, on the other hand, has toosmall an amplitude, it still fails to produce an interferingsynchronising pulse.

The present invention further provides a television receiver adapted foruse in receiving a carrier wave modulated with a composite signalcomprising trains of picture signals interspersed with synchronisingsignals, the receiver having means for separating the synchronisingsignals from the picture signals, wherein the circuits between theaerial, or equivalent input terminals of the receiver, and theseparating means are such that the separation is unaffected byvariations in the picture signals.

Where the separation is effected after the signal modulation has beenextracted by a detector, the receiver according to this invention mayhave a conductive coupling between the detector and the separating pointso that the D. C. component of the modulation is present at theseparating point. Alternatively the separation may be effected at radiofrequency.

The invention will. be described more particularly with reference to asystem for transmitting motion pictures from film but is of course alsoapplicable to systems for transmitting directly from objects.

The invention will be described by way of example with reference to theaccompanying drawings in which Fig. 1 is a diagrammatic representationof a transmitter according to the present invention.

Figs. 2 to 6 are circuit'diagrams showing various forms of re-insertingdevices which may be used in the arrangement of Fig. 1,

Fig. 7 is a. diagrammatic representation of an alternative form oftransmitter according to this invention, and

Figs. 8 and 9 show circuit arrangements of receivers according to thepresent invention.

Referring to the schematic diagram of Fig. 1,

a film I is moved at a uniform speed in a direction normal to thesurface of the paper and is scanned in a series of transverse strips bysuit able means such as a mirror drum 2. Images of the strips are thusswept over an apertured photo-electric cell 3 and generate picturesignals therein. The form of these signals is shown graphically at l,the line 5 representing complete black.

At the end of the scanning of each strip of the picture, a linesynchronising pulse is generated, for example, with the aid of aseparate light source 5 and photo-electric cell I co-operating with themirror of the drum which has just scanned the strip orpreferably asshown with the mirror 8 which will next scan a strip. Similarly, by anyknown or suitable means, a frame impulse is generated at the end of eachcomplete scanning of one frame. During the generation of thesesynchronising signals the light arriving at the photo-cell 3 generatingthe picture signals is suppressed so that the output intensity in thepicture channel corresponds to full black.

This may be done with the aid of a shutter mechanically coupled to thedrum 2 or by render-- ing the edge of the film I opaque.

If desired the synchronising pulses (of wave form indicated by the curve9) may be amplified in an amplifier l0. Similarly the picture signalsmay be amplified if desired in an amplifier II. The synchronisingimpulses are then mixed with the signals in the picture channel so as togive in effect signals in a blacker than black sense, that is to say,the sense of the synchronising signals with respect to a signal levelcorresponding to black is opposite to the sense corresponding toincreased brightness. Care is taken that the synchronising signals areof constant amplitude.

The composite signal, thus constituted, is then passed through aresistance coupled amplifier I2 which on account of the condenser I3 isincapable'of passing direct current signal components and components ofvery low frequency below the frequency of the line synchronisingimpulses.

These components will be referred to herein for simplicity as the D. 0.component. This condenser l3 thus acts as a suppressor of the D. C.component. The signal in the output of amplifier I2 is thus of the formindicated at 14, the oscillations taking place about an electrical zeroline l5 such that the average areas enclosed by the curve [4 above andbelow the line I5 are equal. Thus when the average brightness of thefilm I changes, the distance from the peaks of the maxima I6 constitutedby the synchronising impulses to the line I5 will change.

On account of the particular features of the receivers according to thepresent invention with which the transmitter has to operate, the ratioof synchronising signal amplitude to the maximum picture signalamplitude can be made much 0-5. Any further decrease of thesynchronising signal amplitude is not found to be of great assistancealthough, in cases where the modulation hasbeen in such sense that thesynchronising signals reduce the carrier amplitude, a ratio as low as0-3 has been used successfully. The maxi- 'mum picture signal amplitudeis of course the .omy in transmitter power, and the suitability of thetransmitter to co-operate with varying types of receiver, to radiate allthe components of the original light modulation down to zero frequency,

that is to say including the D. C. component.

For this purpose it is necessary to ensure that in some manner a signalcorresponding to the D. C. component of the light arriving at thephoto-cell in the picture channel is correctly applied to the modulatortubes of the radio frequency transmitter.

For this reason the composite signal is fed to a re-inserting device I1,certain forms of which will be described later, whereby the D. C.component is re-inserted into the signal. The signal may thus be of theform shown at l8. The effect of the re-insertion of the D. C. componentis that,

in spite of changes in the average brightness of,

the film I, the peaks IE will represent a constant voltage and will beconstantly related to the voltage corresponding to picture blackindicated by the line iii. The signal with its D. C. component is thenpassed to a modulator2|l wherein carrier oscillations generated by anoscillator 2| are modulated by the signal. Because the D. C. componentis present, it can be arranged that any given voltage, such as thatcorresponding to black for example, is represented by a fixed value ofcarrier amplitude whatever may be the average value of the picturebrightness. Thus the peaks It may be made to correspond substantially tozero carrier amplitude.

The re-inserting device I! may be of a variety of different forms.Preferably it is of the kind set forth in the copending application Ser.No. 720,205. Certain forms of device according to that specification areshownin Figs. 2 to 6, wherein the input terminals A and output terminalsB correspond to terminals A and B in Fig. 1.

The signals are applied to the terminals A in such a sense that thesynchronising impulses tend to make the grid of the triode 22 positive.The condenser 23 and the resistance 24 are arranged to have a timeconstant which is longer than the intervals between successivesynchronising impulses and shorter than the time constant of any circuitwhich the signals have traversed since they contained their D. C.component, for example shorter than the time constant of condenser l3and leak resistance 25 of Fig. 1.

Assuming first of all that no picture signals are present, synchronisingpulses of the kind shown at 9 in Fig. 1 are applied to the grid of valve22 and the first of these causes grid current to flow and charge thecondenser 23. When the impulse ceases the grid will be more negativethan before and this negative charge will leak away only slowly throughleak 24. The next impulse again caused grid current to flow therebyincreasing the negative charge on the grid after the impulse has ceased.This process continues until a steady state is reached in which gridcurrent only Just flows at the peak of each synchronising impulse. Whenpicture signals are present they make the grid more or less negative anddo not-cause any grid current to flow. They do-not therefore affectappreciably the charge on the condenser 23. Thus, however, the averagevalue of the picture signals may change, the grid of valve 22 will beheld at such a value that the peaks of the synchronising impulses justcause grid current to flow and the signal at the output terminals B willtherefore contain the D. C. cmponent. The re-inserting means will thusbe seen to comprise a unidirectionally conducting device, constituted bythe grid circuit of valve 22, in combination with the circuit 23, 24 oftime constant which is longer than the intervals between successivesignal maxima in one direction, in this case the positive direction. Inthe form of reinserting device shown in Fig. 3, the

condenser 23 and resistance 24 as before constitute a circuit of longtime constant. In this case it is arranged that the synchronising im- Ipulses are in a negative sense and. the picture signals in a positivesense. The necessary change canof course be obtained by the insertion ofan additional stage of thermionic valve ampliflcation. tionallyconducting device and the anode of the diode is connected to a tappingpoint on a potential di.ider 21 bridging a voltage source 28. Thenegativ: maxima, constituted by the synchonising impulses, cause currentto fiow in the diode and after the impulse has ceased the cathode of thediode is more positive than the anode.

The processcontinues with each succeeding impulse until current onlyjust flows in the diode at the peaks of the maxima. In the signals atthe terminals B, therefore, the D. C. component will appear. The datumline of these signals can be adjusted with the aid of the potentialdivider 21.

The circuit of long time constant need not necessarily include acondenser and resistance as in Figs. 2 and 3. For example in the deviceof Fig. 4 it is constituted by an inductance 29 and a resistance 30(together with the resistance of the diode 3| when passing current). Theunidirectionally conducting device is, as in .Fig. 3, the diode 3|.Whereas the terminals A of Figs. 2 and 3 are preferably fed from a lowimpedance amplifier output, the terminals A of Fig. 4 may A diode 26constitutes the unidirecbe fed from a high impedance amplifier output.

In operation, the signals are applied to the re-inserting device of Fig.4 in such a sense that the synchronising impulses tend to make the upperend of inductance 29 more negative. The negative maxima cannot passthrough the rectifier 3| and they therefore charge" the inductance 29,that is to say they initiate in the inductance 29 a flow of currentwhich tends to persist after the impulse has ceased. This persistence ofcurrent flow is equivalent to the discharging of a condenser and theonly path through which it can flow is through the diode 3| and theresistance 30 in series. This current supplies the necessary D. C.component.

a drop of applied voltage thus occurs. The efiect.

of this is that the whole of the missing D. C. component is notre-inserted. In some cases this may not be material. Where it is desiredto obtain more complete re-insertion, means may be provided at somesuitable point for favouring the D. C. signal component (this expressionbeing used as elsewhere in this specification to include low frequencysignal components) relatively to the remainder of the signal frequencyband. One arrangement of this kind is shown in Fig. 5.

The circuit is the same as that of Fig. 2 excepting that across theanode resistance 33 of the valve 22 thereis shunted a circuit comprisinga resistance 34 in series with a condenser 35. The

' circuit 34, 35 serves to favour the low frequencies compared with thehigh frequencies appearing at terminals B because thelatter areby-passed to a larger extent through circuit 34, 35 than the former.

Another re-inserting device favouring the low frequencies is shown inFig. 6. Here the condenser 23 and the resistance 24 function as in Figs.2, 3 and 5. A diode 36 is connected in parallel with the resistance 24.The signals are applied to terminals A-with the synchronising signals ina positive sense. When the steady state is reached the condenser 23 ischarged with the right hand plate negative with respect to the left handplate to such an amount that current just flows through the diode 36 onthe peaks of the maxima. A suitable tapping point on the resistance 24is connected through a resistance 31 to the grid of an amplifying valve38.. A circuit comprising a resistance 39 in series with a condenser 40is connected across thegrid circuit of the valve 38 and serves to favourthe low frequencies at the expense of the high frequencies appliedbetween the grid and cathode of the valve 38.

Instead of using a re-inserting device at H in Fig. 1 it is possible toprovide a circuit capable of passing direct current between thephoto-cell 3 and the modulator 20. This is however usually inconvenientand an alternative arrangement is shown diagrammatically in Fig. '7.Here the circuit to the left of terminals C and D in Fig. 1 is omittedand may be the same as in Fig. 1. The signals containing the D. C.component are fed to an auxiliary modulator 4i and are used to modulatean auxiliary carrier generated at 42. The modulated auxiliary carrier isamplified as desired by amplifier 43 and is then applied to ademodulator 44. The amplified picture and synchronizing signals withtheir D. C. component are then used in modulator 45 to modulate acarrier generated at 46.

Clearly if desired the demodulator and main modulator 44 and 45 can beomitted if desired and the auxiliary oscillator 42 is then arranged tosupply the carrier frequency which is to be radiated.

Since the synchronising signals are purely A. C. in character andcontain no components below the frame frequency, there is no need, inthe case of these signals, to provide D. C. amplification and where aseparate amplifier is used for the synchronising signals, this may be ofthe A. C. type. This may be explained with reference to Fig. l. Thesynchronising signals are shown at 9 and it is desired to superimposethem upon the picture signals 4 in such a way that the synchronisingimpulses occur in the gaps between trains of picture signals where thepicture signals correspond to black. As shown in this figure, this isdone by feeding the impulses through a transformer into the picturesignal channel, which cannot pass any D. C. component with means for,

' value-for example about 3 volts.

of the synchronising signals. In spite of the fact that no D. C. ispassed from the terminals D, D into the picture channel neverthelessimpulses in the primary of the transformer will give rise tocorresponding impulses in the secondary. The impulses shown at 9 are theline impulses and at the end of every frame there occurs a framingimpulse which is usually of the same amplitude asa line impulse but oflonger duration.- These framing signals can equally well 7 be appliedthrough the transformer, and the desired superposition-can be effectedtherefore satisfactorily without a. D. C. coupling because the lowestfrequency of interest in the synchronising signals is the component offrame frequency.

When modulation is carried dut in any of the ways above described sothat the D. C. component is present at the modulating point, theamplitude of the carrier corresponding to black and that correspondingto the peak of a synchronising pulse will remain constant independentlyof any changes in average picture brightness.

It should be noted that, in modulating a' carrier in this way, use canbe made of the lower curved part of the anode current-grid voltagecharacteristics of thermionic valves, that is the part corresponding tolow anode currents, so long i as it is the synchronising signals whichoperate upon these parts of the characteristic and-so long as thesesynchronising signals are initially given a sufilciently great amplitudeso that after their amplitude has been reduced by the curvature of thecharacteristic it has the desired value relatively to the picturesignals.

In one form of receiver according to this invention showndiagrammatically in Fig. '8 the modulated radio frequency signals arereceived on a suitable aerial system and amplified as required in anamplifier indicated at 41. If desired, a supersonic heterodyne receivermay be used.

In any case the radio frequency carrier modulated by the compositesignal (that is the intermediate frequency in the case of a superhete'r'odyne). is applied to the control grid-cathode circuit of a pentode 48arranged to operate as an anode bend detector. The various voltagesrequired for the receiver are preferably derived from some form ofstabiliser as many of these voltages should be maintained as steady aspossible. One suitable known form of stabiliser shownat 49 comprises anumber of electrodes arranged equally spaced apart within an enclosedenvelope containing neon. The pressure within the envelope is arrangedto be such that the striking voltage between the electrodes has thedesired value. In the case illustrated the arrangement issuch that fiveelectrodes are provided with, in operation, a voltage drop of sayseventy volts between each adjacent pair. The stabiliser is arranged inseries with a. resistance 50 across a suitable voltage source, forexample of about 350 volts and provided the current taken is below apredetermined limit the voltage of each electrode will remainsubstantially constant.

Between the negative terminal of the stabiliser and the negativeterminal of-the source is arranged a small resistance 5| which isshunted by a copper-copper oxide rectifier 52 to limit the voltage dropacross this resistance to a desired The resistance is connected in thecontrol grid-cathode circuit of the pentode in such a way as to maintainthe control grid about 3 volts negative relatively to the cathode. Thegrid nearer the con- .trol grid is connected to a variable tapping point63 of a potential divider 53 connected between say the 140 and 210 voltelectrodes of the stabiliser. The outer grid is connected to the cathodeand the anode of the pentode is connected to the 280 volt electrode 54of the stabiliser through'a resistance 55. A variable tapping point onthis last mentioned resistance is connected to the cathode 51 of acathode ray tube shown diagrammatically at 56 (or to an electrode of asodium lamp for example if mechanical scanning means are provided at thereceiver) whilst the control grid 59 of the cathode ray tube (or theother electrode of the sodium lamp) is connected to a variable tappingpoint 65 on the potential divider 53.

The connections to the control grid and cathode .of the cathode ray tubeare suitable for the present circumstances but in some cases may requireto be reversed in order that the grid may be swung in the positivedirection by picture signals corresponding to increased brightness as isobviously required.

The anode of the pentode I6 is also connected directly to the controlgridof a triode 59, the cathode of which is connected to a variabletapping point 65 on the potential divider 53. The

anode of the triode 59 is connected through a resistance 60 to thepositive stabiliser 49 and through a condenser 6| to a frame and linesynchronising signal separator 62.

The last mentioned potential divider tapping point 65 is so adjustedthat current flows in the anode circuit of the triode 59 when no carrieris received and the amplitude of the signals applied to the pentodedetector 48 is arranged to be such that current flow in the triode 59justceases when the carrier amplitude is increased to a valuecorresponding to 'black. No current therefore flows in the triode 59 inresponse to picture signals and the triode acts as a separator valve forseparating the synchronising signals from the picture signals. Thesynchronising signals fed through the condenser 6| to the frame and lineseparator 62 are thus free from picture signals, and because of theconductive coupling between the detector 46 and the separating triode59, the amplitude will remain constant in spite of changes in theaverage picture signal amplitude.

The frame and line signal separator 62 serves to separate linesynchronising impulses from frame impulses. The separator 62 maybe ofany 4 known or suitable kind, for example it may operate by virtue ofthe frequency diflerence between the line and frame impulses.

The line synchronising impulses thus appear at terminals 66 which may beconnected as shown to a device 61 which serves to generate oscillationsof saw tooth wave form under the control of the line impulses. These sawtooth oscillations may then be fed to deflecting coils 66 serving todeflect the ray in'one scanning co-ordinate.

. g Th frame impulses appear at terminals 69 and may be applied to asecond saw tooth wave generator which is not shown and this second-sawtootliwave may be applied to deflecting coils (not shown) serving todeflect the ray inthe co-ordinate at right angles to that of coils 66.

It will be seen that the modulation of the carrier at the transmitterhas been carried out in such a way that black is represented by a flxedvalue of carrier amplitude and at the receiver, by providing a D.coupling between the detector and the separating point, the voltageacross the terminal 54 of the of the synchronising signals grid circuitof the triode corresponding to black" is also independent of the averagepicture amplitude. In this way a substantially constant amplitude ofsynchronising pulses is obtained and the amplitude of these pulses cantherefore be made smaller in relation to the picture signals thanhitherto.

In the case of a mechanical scanner at the receiver, the synchronisingsignals may of course be used to synchronise the mechanical scanning.device.

In another form of receiver according to this invention illustrateddiagrammatically in Fig. 9, the modulated carrier is applied aftersuitable ampliflcation to terminals and therefore across the controlgrid and cathode of a triode 1| arranged as an anode bend rectifier. Theanode is connected through a resistance 12 to the 280 volt electrode ofastabiliser 13 and the cathode is connected toeearth and to the negativeterminal of the stabiliser.

In this case oscillations of saw-tooth wave form are generated forexample in the manner set forth in the specification of British PatentNo. 400,976 by means of discharge tubes of the known type in which theanode-cathode impedance remains high so long as its grid is maintainedat a suitable negative potential relative to its cathode but falls to' alow value when thenegative potential on the grid is suitably reduced--and remains at a low value irrespective of any potentials subsequentlyapplied to the grid until conductively through a resistance 16 to thegrid of the line frequency discharge device 16 and through a low-passfilter represented diagrammatically at 11 to the grid of the framefrequency.

discharge device 15. The cathodes of the devices 14 and 15 are connectedto variable tapping points 8|, 82 on a potential divider 18 arrangedacross the 140 and 210 volt electrodes of the stabiliser 13. Theresistance 50 and the resistance and rectifier 5| and 52 perform thesame functions as in Fig. 8.

The modulated carrier oscillations are also fed to terminals 60 andthereby to a second detector 19 which serves to pass the picture signalsto the cathode ray tube or other device used for reproduction indicateddiagrammatically at 56. Thus a tapping point on the anode resistance 83of valve 19 is connected to the cathode 51 of the cathode ray tube 56and the control electrode 58 of this tube is connected to a variabletapping. point on the potential divider 18.

The connections in the anode circuits of the devices 14 and 15 are notillustrated because they are already well known.

ture amplitudes do not appear as corresponding pulses inthe detectoroutput. Such partial separation is, if the transmitter is radiating theD. C. component of the picture modulation, effected under stabilisedconditions as regards the n. c. component of the modulation, sinceparticular carrier values correwond to particular picture signalamplitudes. It will be seen that separation of this type embodies theessential feature of the invention. It should also be noted that to takefull advantage of separate detectors for synchronising and picturesignals, it should be arranged that each detector operates over the'mostfavourable part of its detection characteristic for the amplitude rangeover which it is required to work. For example the picture detectorshould be most efficient between radio frequency amplitudescorresponding to the range between full black and full white and thesynchronising detector should operate most efiiciently for radiofrequency amplitudes in the range corresponding to blacker than black.By suitably choosing the operating range of a synchronising detector, afurther partial degree of separation can be obtained.

The system above described is arranged, by adjusting the tapping points8| and 82 on the potential divider, so that the discharge devices 14 and15 oscillate at a faster rate than the correct rate when no carrier isbeing received and the arrangement is further made such that thedischarge devices run slower than the correct rate when the receivedcarrier amplitude corresponds to complete black. The synchronisingsignals then serve to hold the discharge devices to the correctfrequencies.

The grids of the discharge devices are driven more negative by thepicture signals and are therefore unaffected by them. Consequently thedischarge devices themselves act as separators and, although preferable,it is not necessary that separation should have been effected previouslyin the amplifier preceding the synchronising signal detector or in thisdetector itself as above described.

The line synchronising impulses are prevented from affecting the framefrequency discharge device by the low-pass filter TI which may consistof series resistances and shunt condensers. The frame frequency pulseson the other hand do not affect the line frequency discharge device I4provided that they are of suitable form. They may for instance be in theform of lengthened line impulses, that is to say a number of lineimpulses (for example three) are lengthened so that they last for nearlythe whole interval between successive line impulses, the abruptcommencements of the line impulses being left un- Y changed. Theseso-called broadened pulses operate the frame frequency discharge devicesatisfactorily but do not produce any other effect upon the linefrequency discharge device 14 than is produced by the ordinary lineimpulses. If desired, however, a high pass filter may be provided in theinput to the line frequency discharge device.

In the above described receiver arrangements the separation ofsynchronising signals from picture signals is effected at leastpartially after detection. The separation may, however, be effectedwholly before detection and the effect is similar to that produced by aD. C. coupling between the detector and the separating point. In otherwords the amplitude of the synchronising signals is independent ofaverage picture amplitude. The sepal tion at radio frequency may beeifected by feeding the modulated radio frequency oscillations to atriode arranged as a limiter. For example the conditions may be madesuch that the valve is swung past its lower .(or if desired upper)bendby the smallest picture signals and remains on the relativelystraight portion only for synchronising signals. Alternatively it may bearranged that the smallest picture signals in crease grid voltage tosuch a point that grid current commences to flow.

Some of the advantages of the present invention can be obtained even ifthe signal used to modulate the carrier at the transmitter has beendeprived of its D. C. component, so long as this component is arrangedto be present at the point at the receiver at which separation iseffected. For example the D. 0. component may be re inserted at thereceiver with reference to recurrent signal maxima or minima, such asthe peaks of the synchronising signals in the manner described inconnection with Figs. 2 to 6. Where this is done the low ratio ofsynchronising signal amplitude to maximum picture signal amplitude maystill be used. There is, however, the disadvantage that a greater rangeof carrier amplitudes is required since the carrier amplitudecorresponding to any particular value of picture brightness varies withchange 0 average picture brightness.

The invention has been described in connection with modulated carriertransmission systems but is of course not limited, in many of itsaspects, to such systems. For example in systems where the connectionbetween the transmitter and receiver is by wire, it is arranged that thecircuits of the system preceding the separating means at the receiverare such that the D. C. component of the modulation is present at theseparating point. Either the circuits may be made conductive or somemeans may be provided for reinserting the D. C. component beforeseparation takes place.

The transmitterdescribed is one employing a mechanical scanning device.The invention can also be applied where other forms of transmitter areused. For example the transmitter may comprise a mosaic photo-electricstructure and may even be of a type in which no D. C. component isgenerated. Where the D. C. component is generated the invention can beapplied as already described. Where the D. C. component is not generatedby the scanner itself, the D. C. component may be generated separately,for example with the aid of a separate photo-electric cell exposed tolight for the whole object to be transmitted, and combined with thepicture signals.

We claim:

1. Television apparatus comprising a transmitter having means forgenerating a carrier oscillation signal, means for generating picturesignals having direct current and low frequency components, said picturesignals being interspersed with synchronizing impulses, the ratio of theamplitude of said synchronizing impulses to the maximum amplitude ofsaid picture signals being substantially less than two, means formodulating said carrier with said signals and impulses including thedirect current and low frequency components, means for transmitting saidmodulated carrier, a receiver adapted to receive the transmittedmodulated carrier, amplitude selection means for separating saidimpulses from said signals. coupling means between oscillation, signalgenerating means for generating picture signals having direct currentand low frequency alternating current components interspersed withsynchronizing impulses, said synchronizing impulses having theirpolarity opposite to that of said picture signals, means to amplify saidpictur'e signals and synchronizing impulses while suppressing the directcurrent and the low frequency alternating current components of saidpicture signals, means for modulating said carrier with said amplifiedsignals and impulses in such a manner that said synchronizing impulsesserve to reduce the carrier amplitude substantially to zero, andcoupling means connected between said signal generating means and saidmodulating means to supply signals representative of the suppresseddirect current and low frequency alternating current components of saidsignal to said modulating means.

3. Television apparatus comprising a transmitter having means forgenerating a carrier oscillation, signal generating means for generatingpicture signals interspersed with synchronizing impulses whose polarityis opposite to that of said picture signals, means for modulating saidcarrier with said signals in such a sense that said synchronizingimpulses serve to reduce the carrier amplitude, coupling means incapableof transmitting direct current and low frequency alternating currentsignal components between said signal generating means and saidmodulating means, and electrical means preceding said modulating meansto supply energy representative of said direct current and low frequencyalternating current signal components.

4. Television apparatus comprising a transmitter having means forgenerating a carrier oscillation, means for generating picture signalsand synchronizing impulses interspersed therewith where the polarity ofthe synchronizing impulses is opposite to that of the picture signals,means for modulating said carrier with said signals and impulses in sucha sense that said synchronizing impulses serve to reduce the carrieramplitude, means incapable of transmitting direct current and lowfrequency alternating current signal components between said signalgenerating means and said modulating means, and means responsive to thepeaks of impulse maxima in one direction to supply said direct currentand low frequency alternating current signal components.

5. Television apparatus comprising a transmitter having means forgenerating carrier oscillations, signal generating means for generatingpicture signals including directcurrent and low frequency alternatingcurrent components interspersed with synchronizing signals whoseamplitudes are greater in the direction of black than the amplitude ofthe picture signals representative of the blackest black of the pictureto be transmitted, means for amplifying said picture and synchronizingsignals while suppressing said direct current and low frequencyalternating current components, means for modulating said carrier byboth of said signals in such a manner that the maximum signals in onedirection serve to reduce the carrier amplitude substantially to zero,and means between said amplifying means 7 and said modulating means tomodulate said carrier oscillations under the control of signalsrepresentative of the suppressed direct current and low frequencyalternating current component 6. Television receiving apparatuscomprising means for receiving a carrier wave modulated with a compositesignal comprising picture signals interspersed with synchronizingsignals, said synchronizing signals rier wave, said demodulatedmeansincludinga thermionic tube having at least a cathode, control electrodeand anode, said picture signals from said synchronizing signals, meansfor coupling said demodulator means to said separator means, a cathoderay tube having a cathode, a control electrode, an' anode, and aluminescent screen for reproducing pictures, and a direct connectionfrom the anode of said thermionic tube to the control electrode of saidcathode ray tube.

7. Television apparatus comprising a transmitter having means forgenerating a carrier oscillation, signal generating means for generatingpicture signals having direct current and low frequency components whichatspaced intervals assume a fixed value, means for generatingsynchronizising impulses, means for superimposing said synchronisingimpulses upon said intervals to form 'a composite signal, amplifyingsaid composite signal without the direct current and low frequencycomponents, means for modulating said carrier with said compositesignal,

means for separating and a circuit between said generating means andsaid modulating means for maintaining peaks of said impulsesproportional to a fixed value of carrier amplitude and for reinsertingthe direct current and low frequency components.

8. Television apparatus comprising a transmitter having means forgenerating a carrier oscillation, signal generating means for generatingpicture signals which at spaced intervals assume a fixed value equal tothe threshold value of the blacker than black region, an amplifierincapable of transmitting direct current for amplifying said picturesignals, means at the output side of said amplifier said carrier by theamplified picture means at the output side of said amplifier responsiveto peaks of said blacker than black values adapted to re-inserting adirect current component into said signals.

9. In a television transmission system comprising means to generatepicture signalling im pulses representative of an image to betransmitted, means to generate synchronizing impulses, means to generatea first source of carrier wave energy, means to modulate the carrierwave energy by both the picture signals, including diproducing picturesignals including the direct current and low frequency componentsrepresentative of an image to be transmitted, producing synchronizingsignals, producing a first source of carrier wave energy, modulating theproduced carrier wave energy oy both the picture and synchronizingsignals, amplifying the modulated carrier wave energy without directcurrent and low frequency components, demodulating the amplified carrierwave energy, producing a second source of carrier wave energy,modulating the carrier wave energy of the second source by thedemodulated energy including the direct current and low frequencycomponents, and transmitting the modulated carrier energy of the secondsource.

11. Television comprising a transmitter having means for generating acarrier oscillation, signal generating means for generating trains ofpicture signals having direct current and low frequency components withintervals between said trains, means to suppress said direct current andlow frequency components, means for modulating said carrier with saidpicture signals whereby an increase in picture brightness is representedby an increase in carrier amplitude, means for modulating said carrierby signals representative of the suppressed direct current and lowfrequency components, means for generating synchronizing impulses, meansfor modulating said carrier in said intervals with said synchronizingimpulses whereby said synchronizing signals reduce the amplitude of saidcarrier, means to linearly amplify the demodulated picture carrier wave,and means to non-linearly amplify the synchronizing demodulated carrierwave.

12. In the method of transmitting television signals, the steps ofgenerating signalling energy representative of an object to betransmitted, generating synchronizing energy, combining the signallingenergy with the synchronizing energy in opposite phase relation,removing energy components from the combined signalling andsynchronizing energies representative of the average value of thecombined energies, generating carrier wave energy, modulating thecarrier energy by the combined signalling and synchronizing energiesfrom which the energy components representative of the average value ofthe combined energies have been removed while regulating the averageamplitude of the carrier wave energy by the removed energy, andtransmitting the modulated regulated carrier wave energy.

13. A television transmitting system comprising means for generatingsignalling energy representative of an object to be transmitted, meansfor generating synchronizing energy, means for combining the signallingenergy with the synchronizing energy in opposite phase relation, meansfor supressing the energy representative of the average value of thegenerated signalling energy, means for deriving energy representative orthe average value of the combined synchronizing and signalling energy,means for generating carrier wave energy, means for modulating thecarrier energy by the combined signalling and synchronizing energies andfor regulating the average amplitude of the carrier wave energy by thederived energy, and means for transmitting the modulated regulatedcarrier wave energy.

14. A television, transmitting system comprising means for generatingsignalling energy representative of an object to be transmitted, meansfor generating synchronizing energy, means for combining the signallingenergy with the synchronizing energy in anti-phase relation, means forremoving energy components from the combined signalling andsynchronizing energies representative of the average value of thegenerated signaling energy, means for generating carrier wave energy,means for modulating the regulated carrier energy by the combinedsignalling and synchronizing energies from which the energy componentsrepresentative of the average value of the generated signalling energyhave been removed and for regulating the average amplitude of thecarrier wave energy by energy representative of the removed energycomponents,.

and means for transmitting the modulated regulated carrier wave energy.

15. Television apparatus comprising a transmitter having means forgenerating carrier oscillations, signal generating means for generatingpicture signals including direct current and low frequency alternatingcurrent components, means for generating synchronizing signals, meansfor combining the picture signals and the synchronizing signals so thatmaximum amplitude of the picture signal is of opposite polarity to themaximum amplitude of the synchronizing signal, means for amplifying thecombined picture signals and synchronizing signals while supressing thedeveloped quency alternating current components, means for modulatingthe generated carrier oscillations by both of said picture andsynchronizing signals in such manner that the maximum intensity signalsin one direction serve to increase the carrier amplitude substantiallyto amaximum and means between said amplifying means and said modulatingmeans for reinserting signals representative of the suppressed directcur rent and low frequency alternating current components to control themodulation of the said carrier oscillations.

PETER WILLIAM WILLANS. WILLIAM SPENCER PERCIVAL. ERIC LAWRENCE CASLINGWHITE. I-IILDA E. F. OSBORNE, Administratria: of the. Estate of CharlesPercy Osborne, Deceased.

direct current and low fre-

